Coordinately Targeting Cell-Cycle Checkpoint Functions in Integrated Models of Pancreatic Cancer.

Abstract

Cancer cells often have deficiencies in cell-cycle control mechanisms and could be dependent on specific cell-cycle checkpoints to maintain viability. Because of the documented role of <i>KRAS</i> in driving replication stress, we targeted the checkpoint governing DNA replication using CHK1 kinase inhibitors in pancreatic ductal adenocarcinoma (PDAC) models and examined mechanisms of resistance. Single-agent efficacy of CHK1 inhibition was investigated in established and primary PDAC lines. Drug screening was performed to identify cooperative agents. <i>In vitro</i> and <i>in vivo</i> studies were employed to interrogate combination treatment efficacy and mechanisms of resistance. Many PDAC models evade single-agent inhibition through mechanisms that allow S-phase progression with CHK1 inhibited. Gene expression analysis revealed FOXM1 as a potential marker of CHK1 sensitivity and defined a form of pancreatic cancer with poor prognosis. Drug screen analysis identified WEE1 as a cooperative agent with CHK1 and was effective in cell culture. <i>In vivo</i> experiments validated the combination efficacy; however, resistance could evolve. Resistance was due to selection of a stable subclone from the original PDX tumor, which harbored high baseline replication stress. <i>In vitro</i> analysis revealed that gemcitabine could eliminate viability in the resistant models. The triplet regimen of gemcitabine, CHK1, and WEE1 inhibition provided strong disease control in all xenograft models interrogated. These results demonstrate the therapeutic resiliency of pancreatic cancer and indicate that coordinately targeting cell-cycle checkpoints in concert with chemotherapy could be particularly efficacious.